This invention relates to a process for manufacturing organic silazane polymers which are suitably used as precursors for ceramic materials, and also to a process for manufacturing ceramics from the organic silazane polymers.
Great interest has been currently shown in ceramics as materials which have excellent properties, such as heat resistance, abrasion resistance, and high-temperature strength, i.e., physical strength at high temperatures. However, because of the hardness and poor chip resistance, such ceramics are very difficult to machine and shape. Methods used to manufacture shaped ceramic articles from such materials include molding a fine powder of the ceramic material into a desired form, such as by compression, followed by sintering, or a "precursor" method in which an organic polymer, serving as a preceramic material, is first melted or dissolved in a solvent, fabrication into a desired form, and sintered to render the polymer inorganic. The main advantage of the precursor method is that ceramic products having complex shapes can be formed, e.g., products in forms, such as fibers and sheets, which cannot be obtained with the fine powder sintering method.
Materials generally used as such ceramics include SiC and Si.sub.3 N.sub.4. These have attracted considerable attention because of their good high temperature properties. For example, SiC has a high heat resistance and a high-temperature strength, and Si.sub.3 N.sub.4 has a high thermal shock resistance and a high fracture toughness. Accordingly, various proposals have been made for processes of producing SiC-Si.sub.3 N.sub.4 ceramics and also for processes of producing organic silicon precursors using the precursor method. These processes and their disadvantages are described in paragraphs (1) to (7) below. Incidentally, infusibilization herein defined is to render an organic substance unable or difficult to melt at temperatures higher than its melting point without turning it inorganic. Sintering herein defined is to turn an organic substance inorganic by heating it at high temperatures.
(1) U.S. Pat. No. 3,853,567 discloses a process of obtaining SiC-Si.sub.3 N.sub.4 ceramics wherein chlorosilanes and amines are reacted and subsequently heated at high temperatures to obtain carbosilazanes. These are then spun, infusibilized, and sintered at high temperatures of from 800.degree. to 2000.degree. C. However, the high temperatures required of from 520.degree. to 650.degree. C. to obtain the carbosilazanes renders this process very difficult to adopt industrially. In addition, this process is disadvantageous in that the yield of inorganic ceramic materials from the carbosilazanes, is low, e.g., about 55%. In the Examples of this U.S. Patent specification only methyltrichlorosilane and dimethyldichlorosilane are exemplified as the chlorosilanes, and methylamine as the amines.
(2) U.S. Pat. No. 4,097,294 teaches the conversion of various silicon-containing polymers into ceramic materials by pyrolysis. However, it discloses only one example of a silazane polymer and the ceramic yield is as low as 12% at best. Although this United States patent specification indicates that ceramic materials may be formed into fibers or thin films, this is merely a suggested possibility. In fact, little mention is made of the moldability and processability of polymers, which is considered to be among the most important aspects of the precursor method.
(3) Numerous methods for the production of silazane polymers have been disclosed. For example, by the reaction between chlorodisilanes and disilazanes shown in U.S. Pat. No. 4,404,153; by the reaction between chlorosilanes and disilazanes shown in U.S. Pat. No. 4,312,970; by the reaction between chlorodisilanes and ammonia disclosed in U.S. Pat. No. 4,395,460; and by the reaction between trichlorosilane and disilazanes disclosed in U.S. Pat. No. 4,540,803, respectively. Moreover, U.S. Pat. No. 4,535,007 discloses the production of silazane polymers wherein metal halides are added to chlorosilanes and disilazanes, and U.S. Pat. No. 4,482,689 discloses another method wherein metal halides are added to chlorodisilanes and disilazanes. It is stated in each of these references that the respective silazane polymers may be converted to ceramic materials by pyrolysis. However, the ceramic yields for these silazane polymers are only 50 to 60 wt %. Also, as in the patent specification referred to in (2) above, none of these references describes in detail the moldability and processability of the polymers, which are most important aspects in the precursor method. In particular, most of the references do not provide any examples for making ceramic fibers, and those showing examples of ceramic fibers do not refer to the strength of the ceramic fibers. Only U.S. Pat. No. 4,482,689 contains a description of the strength of the fiber; however, according to the disclosure, the ceramic fibers have a low tensile strength such as 53 kg/mm.sup.2 or 63 kg/mm.sup.2.
(4) U.S. Pat. No. 4,482,669 describes a process for preparing silazane polymers which comprises reacting ammonia with an organosilicic compound of the formula, ##STR2## to obtain an ammonolysis product and subjecting the product to condensation by dehydrogenation with alkali metal hydrides or alkaline earth metal hydrides. It is stated that the polymers obtained in this process can be controlled to have various forms from oils to solids having extremely high melting points, depending on the degree of condensation by dehydrogenation. However, when a polymer melt is molded or processed to prepare, for example, a continuous fiber by melt spinning, it is necessary that the polymer have a certain degree of polymerization and be thermally stable. In the above process, however, the polymer obtained will be in the form of a solid having an extremely high melting point, unless the polymerization is stopped halfway. In order to obtain a conveniently fusible polymer, the reaction time, reaction temperature, amounts of catalyst and solvent, etc., have to be controlled precisely. However, such control may be very difficult to achieve and may not be reproducible. The polymers obtained by the process are not thermally stable, and have the disadvantage that gel-like substances are formed. In view of the above two problems, this process may not be considered suitable as an industrial process of manufacturing silazane polymers.
(5) U.S. Pat. No. 4,595,775 describes a process for preparing a silazane polymer which comprises producing a cyclic silazane from a reaction between a compound of the formula: ##STR3## and monomethylamine, followed by reacting the cyclic silazane with ammonia. In this publication, it is stated that this polymer is suitable for use as a material for chemical vapor deposition. However, neither the physical properties of the polymer nor the ceramic yield are described.
(6) K. A. Andrianov and others reported in J. Organamet. Chem. 3, 129-137 ('65), that it is possible to obtain a polymer by the following procedure: react dimethyldichlorosilane and ammonia to produce a silazane compound, and polymerize the silazane compound in the presence of KOH as the catalyst. These reactions are shown below.
Baking the polymer to turn it into ceramic is not mentioned; however, the present inventors consider that this polymer when molded into various shapes will be poor in infusibility, wherefore this polymer, having no radicals that are capable of crosslinking, must receive a considerable amount of highly energized irradiation of electron beam, .gamma.-ray, etc. to become infusible. Also, another inconvenience with this polymer is that its ceramic yield after baking is as small as 35-40%. ##STR4##
(7) A. A. Zhdanov and others reported in Polym. Sci, USSR 23 (6) 1429-1438 ('81) that a silazane polymer is obtained by reacting trimethyltrivinylcyclotrisilazane with KOH (1%) at 200.degree. C. A disadvantage of this polymer lies in that it undergoes prompt polymerization when heated to temperatures above 200.degree. C. to produce a polymer which is insoluble and infusible, and therefore not moldable. Also, this report does not mention conversion of the polymer to a ceramic.
As is apparent from the foregoing description, hitherto proposed silazane polymers are not appropriate for industrial use as preceramic materials. In addition, these polymers were found to exhibit poor moldability and processability as precursors for ceramic fibers, and poor ceramic yield as well. Ceramic products, e.g., ceramic fibers obtained from the known preceramic polysilazane materials were found to have relatively poor physical properties such as strength, modulus of elasticity and the like.